This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2000-262585 filed on Aug. 31, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to a hydraulic brake device for a vehicle. More particularly, the present invention pertains to a hydraulic brake device for a vehicle which includes a dual brake pressure supply circuit for supplying brake pressure from a hydraulic pressure generating device to wheel cylinders provided respective wheels of the vehicle.
Various types of the hydraulic brake devices are known. One type of hydraulic brake device generally used includes a dual brake pressure supply circuit for supplying brake pressure from a hydraulic pressure generating device to wheel cylinders provided on each wheel of a vehicle, a master cylinder serving as a hydraulic pressure generating device, and a booster for assisting the actuation of the master cylinder. The booster is a so called servo unit and assists the operation of the master cylinder in response to brake pedal operation.
Known types of boosters include negative pressure boosters or vacuum boosters that are driven by negative pressure supplied from an intake manifold of the engine of the vehicle and hydraulic pressure boosters driven by an auxiliary hydraulic pressure source. In this type of hydraulic pressure brake device, the master cylinder including a piston with a larger diameter is preferable for reducing a brake pedal stroke. However, if the master cylinder including the piston with a larger diameter is used, the booster is required to increase the assisting power for assisting the operation of the master cylinder and also assure the braking force when the booster fails.
U.S. Pat. No. 5,029,951 (corresponding to a Japanese Patent Application published on Apr. 6, 1990 as Toku-Kai-Hei 2 (1990)-95966) discloses a hydraulic brake device provided with a hydraulic booster for assisting the operation of a master cylinder in response to a brake pedal operation by an output pressure of a power hydraulic pressure source (auxiliary hydraulic pressure source). This hydraulic brake device further includes a power piston having a diameter larger than that of a master piston, and a sealed chamber for hydraulically connecting the power piston and the master piston. The master piston is directly actuated in response to brake pedal depression when the booster fails. Also a one-way valve is interposed between a boost pressure chamber in the hydraulic booster and the sealed chamber so that the power piston and the master piston may be mechanically connected when the boost pressure in the hydraulic booster is not increased in response to the brake pedal depression. Accordingly, this hydraulic brake device is able to reduce a brake pedal stroke and avoid the above problems.
However, even with the hydraulic brake device disclosed in U.S. Pat. No. 5,029,951, the master cylinder is connected with the hydraulic booster and the sealed chamber is further added to the hydraulic booster. This increases the axial length of the overall device. Contributing also to this increased axial length is that the hydraulic brake device disclosed in U.S. Pat. No. 5,029,951 includes a tandem master cylinder connected with two brake pressure supply circuits. Accordingly, mounting the hydraulic brake device on the vehicle is difficult due to a limited space in the engine compartment of the vehicle.
A need thus exists for an improved hydraulic brake device for a vehicle that supplies brake pressure to wheel cylinders provided on each wheel of a vehicle through first and second brake pressure circuits. A need also exists for a hydraulic brake having a hydraulic pressure generating device and an assisting device having a reduced axial length for reducing a brake pedal stroke.
According to an aspect of the present invention, the hydraulic brake for a vehicle comprises a reservoir for storing brake fluid, first and second sets of wheel cylinders each provided on a wheel of the vehicle, and a hydraulic pressure generating device. The hydraulic pressure generating device includes a first piston for increasing a pressure of the brake fluid supplied from the reservoir into a first pressure chamber to supply brake pressure to the first set of wheel cylinders in response to brake pedal depression, and a second piston provided at the rear of the first piston and having a diameter larger than that of the first piston for increasing the pressure of the brake fluid supplied from the reservoir into a second pressure chamber between the first piston and the second piston to supply brake pressure to the second set of wheel cylinders in response to brake pedal depression. An assisting device is provided rearwardly of the second piston for assisting the actuation of the first and second pistons in response to the brake pedal depression. A valve device hydraulically closes the second pressure chamber from the reservoir when the actuation of each of the first and second pistons is assisted by the assisting device, and establishes hydraulic communication between the second pressure chamber and the reservoir when the assisting operation of the assisting device is not carried out. A first brake pressure circuit hydraulically connects the first pressure chamber with the first set of wheel cylinders and a second brake pressure circuit hydraulically connects the second pressure chamber with the second set of wheel cylinders. The assisting device can be either a negative pressure or vacuum booster or a hydraulic pressure booster.
When the first piston is assisted through the second piston by the assisting device, the second pressure chamber is hydraulically sealed by the valve device and then the second piston and the first piston are hydraulically connected. When the assisting operation of the assisting device is not carried out, the second pressure chamber becomes atmospheric pressure by opening of the valve device, and the second piston and the first piston are mechanically connected.
Preferably, the first brake pressure circuit connects the first pressure chamber with the first set of wheel cylinders provided on a pair of wheels located at the front of the vehicle and the second brake pressure circuit connects the second pressure chamber with the second wheel cylinders provided on a pair of wheels located at the rear of the vehicle. Also, the valve device is preferably located in the second piston and is located at the axial center of the second piston.
In accordance with another aspect of the invention, a hydraulic brake device for a vehicle includes a reservoir for storing brake fluid, a first set of wheel cylinders operatively associated with a pair of front wheels of the vehicle, a second set of wheel cylinders operatively associated with a pair of rear wheels of the vehicle, and a hydraulic pressure generating device that includes a first pressure chamber adapted to be communicated with the first set of wheel cylinders, a second pressure chamber adapted to be communicated with the second set of wheel cylinders and positioned rearwardly of the first pressure chamber, a first piston which increases a pressure of the brake fluid supplied from the reservoir to the first pressure chamber during movement of the first piston, and a second piston positioned rearwardly of the first piston and having a diameter larger than that of the first piston which increases the pressure of the brake fluid supplied from the reservoir to the second pressure chamber during movement of the first piston, the second pressure chamber being located between the first and second pistons. An assisting device positioned is rearwardly of the second piston for assisting actuation of the first and second pistons, and a valve device prevents hydraulic communication between the second pressure chamber and the reservoir when the actuation of each of the first and second pistons is assisted by the assisting device, and establishes hydraulic communication between the second pressure chamber and the reservoir non-operation of the assisting device. A first brake pressure circuit hydraulically connects the first pressure chamber with the first set of wheel cylinders, and a second brake pressure circuit hydraulically connects the second pressure chamber with the second set of wheel cylinders.